1. Field of the Invention
The present invention generally relates to a semiconductor optical modulator, a Mach-Zehnder optical modulator employing the same, and a method of manufacturing a semiconductor optical modulator. More specifically, the present invention relates to a semiconductor optical modulator, a Mach-Zehnder optical modulator employing the same, and a method of manufacturing a semiconductor optical modulator that are suitable for high-speed baseband communication.
2. Description of the Related Art
As one of today's known devices that modulates continuous light based on waveforms of input electric signals, there is a Mach-Zehnder optical modulator made of LiNbO3 (lithium niobate). One such Mach-Zehnder optical modulator 1000 is schematically shown in plan view (FIG. 1).
Referring to FIG. 1, a Mach-Zehnder optical modulator 1000 includes a phase modulator 1100, two optical couplers 1003 and 1005, and a pair of optical waveguides 1004 constituting an interference system.
FIG. 2 shows a cross-sectional view of the phase modulator 1100. Referring to FIG. 2, the phase modulator 1100 (made of LiNbO3), which is a conventional type, includes a signal electrode 1108 and a ground electrode 1109, both Au-plated (or gold-plated), and a substrate 1102 made of LiNbO3 as an insulating material. Each of the Au-plated electrodes 1108 and 1109 is laminated on a silica (SiO2) film disposed between the Au-plated electrodes 1108 and 1109 and the substrate 1102. Further, an optical waveguide core 1103 is formed in the LiNbO3 substrate 1102 by a Ti-diffusion process, etc.
In the above stated structure, when a voltage is applied between the Au plated signal electrode 1108 and the Au plated ground electrode 1109, the intensity distribution of the resulting electric field between the electrodes 1108 and 1109 is generally uniform. The distributed electric field is partly applied at the Ti-diffused optical waveguide core 1103, thereby causing a refractive index gradient on the incident light in this region.
The sectional configuration as shown in FIG. 2, is generally designed as a traveling-wave electrode type structure. By matching the input impedance to 50 ohms, reflection to the drive circuit (which is in the input side) is reduced, even when the high-frequency electric signal is input. However, when the LiNbO3 material system is employed in that type of device as shown in FIG. 2, the typical device length is around 40 mm, which is very long.
Aside from the above mentioned type of optical modulator that employs LiNbO3 material system there is another type of optical modulator that utilizes semiconductor material. When semiconductor material is employed, the device length is from several dozen micrometers through several millimeters. It means that the device length of optical modulators made of semiconductor material is shorter than those optical modulators made of LiNbO3. Further, the employment of semiconductor material results in a capability of integration with a light source that is also made of semiconductor material, for example, a laser diode (LD).
A structure of an optical modulator made of semiconductor (or semiconductor optical modulator) 1110 is schematically shown in FIGS. 3 and 4. First, referring to FIG. 3, the semiconductor optical modulator 1110 includes an optical-waveguide-core layer 1113, an electrically-conductive semiconductor layer 1115, an electrically conductive substrate 1112, a metal electrode (signal) 1118 and a metal electrode (ground) 1119. The semiconductor layer 1115 and the substrate 1112 are located on the opposite sides of the core layer 1113. Further, the electrode (signal) 1118 and the electrode (ground) 1119 are located so as to sandwich the combination of the semiconductor layer 1115, the core layer 1113 and the substrate 1112. More specifically, the electrode (signal) 1118 is located on the semiconductor layer 1115 and the substrate 1112 is located on the electrode (ground) 1119. In this structure, high-frequency electric signals from a high-frequency electric signal source 1114 are input into the electrodes 1118 and 1119, thereby providing a predetermined modulation for the light (continuous light).
This type of optical modulator 1110 may be utilized as an absorption-type, single-unit optical modulator. Alternatively, it may be used as a phase modulator part of a Mach-Zehnder optical modulator, as shown in FIG. 1.
FIG. 4 shows a layered structure as seen in another section perpendicular to the optical axis of the optical modulator 1110 shown in FIG. 3. The layered structure shown in FIG. 4 includes an n-type InP layer (n-InP cladding layer) 1122, a thin i-InGaAsP optical-waveguide core layer 1123 and a p-type InP layer (p-InP cladding layer) 1125, wherein the n-InP layer 1122 and the p-InP layer 1125 sandwich the i-InGaAsP optical-waveguide-core layer 1123. The i-InGaAsP optical waveguide core layer 1123 forms the optical waveguide core layer 1113.
The i-InGaAsP optical waveguide core layer 1123 in the above structure is an undoped one and, therefore, it can be regarded as a substantially insulating body. In contrast, the n-type InP layer 1122 and the p-type InP layer 1125 (which sandwich the i-InGaAsP optical waveguide core layer 1123) are substantially conductive bodies. Therefore, when a reverse bias voltage is applied between a pair of electrodes 1128, 1129 (both being gold-plated and situated respectively on top of and under the element structure, i.e., respectively on the top face of the p-InP layer 1125 and on the bottom face of the n-type InP layer 1122), the electric field takes place in a concentrated manner at the undoped i-InGaAsP optical waveguide core layer 1123 and therefore, the electric field within the p-InP layer 1125 and the n-type InP layer 1122 is almost non-existent (or negligible).
Consequently, the i-InGaAsP optical waveguide core layer 1123 can be regarded as a parallel plate capacitor that has p-type and n-type semiconductor electrodes. The predetermined thickness (shown in FIG. 4) of the i-InGaAsP optical waveguide core layer 1123 is 0.5 micrometers or less, which is very thin in order to reduce the driving voltage. Therefore, this capacitor has a large electric capacitance per unit length. As a result, the semiconductor optical modulator 1110, having a layer structure as shown in FIG. 4, has a very large capacitance per unit length thereof.
The structures as shown in FIGS. 3 and 4 are in accordance with a lumped parameter device concept. In contrast, a semiconductor optical modulator that has a traveling-wave electrode type structure is also known, in addition to those employing LiNbO3. This type of optical modulator is also designed in such a manner to be matched toward the 50-ohm input impedance. However, the device length of such an optical modulator is about 1 mm, which is longer than that of the above lumped-parameter type semiconductor optical modulator 1110.
Schematic views of the device structure of this traveling wave electrode type optical modulator are shown in FIGS. 5 and 6. As is easily seen, the layered structure shown in the sectional view of FIG. 6 is quite similar to the structure of the lumped-parameter type semiconductor optical modulator 1110 shown in FIG. 4 in that it contains a structure that includes electrically-conductive p-type and n-type InP layers that sandwich a thin optical waveguide core layer made of undoped InGaAsP.
In a semiconductor optical modulator 1130 shown in FIG. 6, a 0.5-micrometer undoped InP layer (i-InP layer 1143a), which can be regarded substantially as an insulating body, is formed under the i-InGaAsP optical waveguide core layer 1143b. Therefore, the electric field caused by the driving voltage on the device is prevented from concentrating on the i-InGaAsP optical waveguide core layer 1143b. As a result, there is a relatively weak electric field uniformly distributed in both the i-InGaAsP optical waveguide core layer 1143b and the i-InP layer 1143a. Because of this layered structure, the electric capacitance per unit length of the semiconductor optical modulator 1130 is smaller, as compared to those without undoped InP layers.
Further, the input impedance is largely dependent on the capacitance per unit length of the sectional layered structure and has a tendency to become smaller when the sectional capacitance becomes larger. Therefore, in the structure as shown in FIG. 4, which has nothing between the i-InGaAsP optical waveguide core layer 1123 and the n-type InP layer 1122, the capacitance becomes very large, which makes the input impedance too small. In contrast, in the structure shown in FIG. 6, the i-InP layer 1143a is formed between the i-InGaAsP optical waveguide core layer 1143b and an n-InP cladding layer 1142 for the purpose of reducing the capacitance and achieving the relatively desirable input impedance value, which is, for instance, in the neighborhood of 50 ohms.
As a matter of fact, however, the capacitance of the semiconductor optical modulator having the layered structure as stated above is still too large, which causes the problem that the input impedance becomes too small, as seen in other conventional semiconductor optical modulators. This is the cause of a problem, for example, where an impedance matching must be made for a drive circuit for a device configured in a 50-ohm-based system, and the semiconductor optical modulator is not capable of being matched to the 50-ohm input impedance. In this type of problem, the high frequency electric signals that are provided for the purpose of optical modulation are returned to the drive circuit due to reflection phenomena.
One idea of alleviating this problem is to reduce the device length of the semiconductor optical modulator so as to reduce the capacitance. With a configuration based on such an idea, however, there arises a problem: the amount of light-absorption change or light-wave-phase change tends to decrease under a fixed driving voltage. In order to compensate for such decrease, the driving voltage must be made higher.
In contrast, the driving voltage of a traveling wave electrode type semiconductor optical modulator can be reduced because of its long device length. It is, however, still practically difficult to design a traveling wave electrode type semiconductor optical modulator that achieves the around 50-ohm input impedance. This is because the capacitance per unit length is still too large, even in the sectional layered structure that has the i-InP layer 1143a as an insulating body under the i-InGaAsP optical waveguide core layer 1143b. 
Therefore, there is also a problem wherein the high frequency electric signals are returned from the device because of reflection phenomena due to the input impedance being less than 50 ohms. In fact, according to a calculation employing the finite element method, etc., the input impedance of the sectional layered structure shown in FIG. 6 is around 25.5 ohms, which clearly shows the existence of the above stated problem because this is far from achieving 50 ohms.
FIG. 7 shows, via a dotted line, the s11 reflection characteristics obtained when high frequency electric signals are input into the semiconductor optical modulator 1130 illustrated in FIG. 6. The s11 reflection characteristics indicate the ratio of the output electric signal intensity and the input electric signal intensity when electric signals output from a port 1 are returned to the same port 1 due to reflection phenomena.
Referring to FIG. 7, the s11 reflection characteristics of the semiconductor optical modulator 1130 illustrated in FIG. 6 become worst (i.e., minus 5.3 decibel) at the frequency value of 17 GHz, where a large proportion of the high frequency electric signal provided thereto is not properly input and is returned toward the drive circuit due to reflection.
A possible way of solving this problem is to increase the thickness of the i-InP layer 1143a in FIG. 6. However, with a configuration based on such an idea, there arises a problem: the intensity of the electric field applied to the undoped InGaAsP layer of the i-InGaAsP optical waveguide core layer 1143b further decreases under a fixed driving voltage, which means that its driving voltage must be increased to a much higher level in order to provide the required intensity of the electric field.
In short, according to the conventional technologies about a semiconductor optical modulator, regardless of whether it is a lumped parameter type or a traveling wave electrode type, it is difficult to achieve both the reduction of return of the high frequency electric signal from the device occurring due to reflection phenomena and the reduction of the required driving voltage at the same time.